The conventional AlGaInP LED, as shown in FIG. 1, has a double heterostructure (DH), which is consisted of a N-type GaAs substrate 3 and a plurality of layers sequentially formed thereon, wherein the layers are: an N-type (AlxGa1-x)0.5In0.5P lower cladding layer 4 with an Al composition of about 70%˜100%, an (AlxGa1-x)0.5In0.5P active layer 5, a P-type (AlxGa1-x)0.5In0.5P upper cladding layer 6 with an Al composition of about 70%˜100% and a P-type high energy gap GaAsP, InGaP, AlGaP, GaP, or AlGaAs current spreading layer 7. The emitting wavelength of the conventional LED structure can be changed by adjusting the composition of the active layer 5 to a wavelength from 650 nm red to 555 nm pure green. One disadvantage of the conventional LED is that, when the light generated by the active layer 5 is emitted downward to the GaAs substrate 3, the light is absorbed by the GaAs substrate 3 due to a smaller energy gap of the GaAs substrate 3. Accordingly, the light-output performance of the LED is greatly reduced.
Some conventional LED technologies have been disclosed to prevent the light from being absorbed by the substrate. However, these conventional technologies still have some disadvantages and limitations. For example, Sugawara et al. disclosed a method, which has been published in Appl. Phys. Lett. Vol. 61, 1775-1777 (1992), that adding a distributed Bragg reflector (DBR) layer onto the GaAs substrate so as to reflect the light emitted downward to the GaAs substrate for decreasing the light absorbed by the GaAs substrate. However, because the DBR layer only reflects the light almost normal to the GaAs substrate, its efficiency is not very great.
Kish et al. disclosed a wafer-bonded transparent-substrate (TS) (AlxGa1-x)0.5In0.5P/GaP light emitting diode [Appl. Phys. Lett. Vol. 64, No. 21, 2839 (1994); Very high-efficiency semiconductor wafer-bonded transparent-substrate (AlxGa1-x)0.5In0.5P/GaP]. This TS AlGaInP LED is fabricated by growing a very thick (about 50 μm) P-type GaP window layer with the use of hydride vapor phase epitaxy (HVPE). Before bonding, the P-type GaAs substrate is selectively removed by using chemical mechanical polishing and etching techniques. The exposed N-type (AlxGa1-x)0.5In0.5P cladding layers are subsequently wafer-bonded to 8 mil-10 mil thick N-type GaP substrate. The resulting TS AlGaInP LED exhibits the improvement in light output twice as much as the absorbing substrate (AS) AlGaInP LED. However, the fabrication process of TS AlGaInP LED is too complicated. Therefore, it is difficult to manufacture these TS AlGaInP LEDs in high yield and low cost.
Horng et al. reported a mirror-substrate (MS) AlGaInP/metal/SiO2/Si LED fabricated by wafer-fused technology [Appl. Phys. Lett. Vol. 75, No. 20, 3054 (1999); AlGaInP light-emitting diodes with mirror substrates fabricated by wafer bonding]. They used AuBe/Au as the adhesive to bond the Si substrate and LED epilayers. However, the luminous intensity of these MS AlGaInP LEDs is about 90 mcd with 20 mA injection current, and is still 50% lower than the luminous intensity of TS AlGaInP LED.